WO1991017558A1

WO1991017558A1 - Improvements relating to thermally-responsive devices

Info

Publication number: WO1991017558A1
Application number: PCT/GB1991/000692
Authority: WO
Inventors: Robert Andrew O'neill; Alan Andrew Waggott
Original assignee: Otter Controls Limited
Priority date: 1990-04-30
Filing date: 1991-04-30
Publication date: 1991-11-14
Also published as: GB9109239D0; GB2243686A; AU7780591A; GB9009620D0

Abstract

A thermally-responsive differential expansion device is similar to a bimetallic device but has its plural layers formed of synthetic plastics materials having lower thermal conductivity whereby the device can sustain a greater temperature differential across its thickness (eg. 10 °C) than the negligible differential that a high thermal conductivity bimetal can sustain. Such a 'biplastics' device can be arranged to be insensitive to the constantly rising temperature as water is heated in a kettle since the device will maintain a generally constant temperature differential, but will respond to the onset of boiling when the temperature differential across the device will disappear. Enhanced effects are obtained by use of fibre reinforced plastics materials with the fibres differently oriented in the different layers, so as to take advantage of the dependency of the coefficient of linear expansion upon fibre orientation. Such 'biplastics' devices provide advantages in kettle controls for example since the sensor has only to be mounted on the outside of the kettle wall and need not be exposed to live steam to operate as a boiling control.

Description

IMPROVEMENTS RELATING TO THERMALLY-RESPONSIVE DEVICES

Field of the Invention:

This invention concerns improvements relating to thermally-respohsive devices and has particular, though not exclusive, application to thermally- responsive devices for use in protective controls for appliances such as for example water boiling vessels.

Background of the Invention;

Bimetallic elements are well known thermally- responsive devices in which the different metal constituents of the bimetal have different coefficients of thermal expansion so that a temperature change will result in a change in the physical configuration of the device as one metal layer of the device undergoes a greater or lesser expansion or contraction than the other. Various configurations of bimetallic elements are known, ranging from a simple rectangular bimetal blade and such like so-called creep bimetals to snap-acting bimetals such as the well known Otter blade of GB 657434 and the dished bimetallic disc. Specially shaped bimetal blades are known which service special purposes, such as for example the pear shaped snap- acting bimetallic blade of GB 2124429 which is specially adapted for use in a current sensitive electrical switch.

Many different forms of protective controls for water boiling vessels are known which incorporate bimetallic device. Element protector devices such as that disclosed in GB 2194099 for example are designed to respond to an overtemperature situation arising in the heating element of an electrically heated appliance by switching off the supply of electricity to the heating element, and steam controls such as that disclosed in GB 2212664 for example are known which respond to the generation of steam when water boils in a water heating appliance by switching off the appliance.

While bimetallic devices have seen wide application in many fields they do however have their limitations. Disclosed in GB 2185161 is an electronic control for a water boiling vessel in which boiling is sensed by sensing the accompanying change in the rate of temperature rise of the water being heated. Such a rate of change sensing control is advantageous in that it does not rely upon the impingement upon a temperature sensitive element of steam generated when water boils in a vessel, and thus does not give rise to special requirements as regards the ducting of steam to the thermally-responsive element and the isolation of electrical parts from the steam and from water condensed therefrom. With a rate sensitive control as disclosed in GB 2185161 for example it is necessary only to arrange a thermistor in good thermal contact with the water in the vessel, which can readily be done by placing the thermistor at an appropriate position. However, electronic controls for water heating vessels are generally more expensive than bimetallic controls.

Object and Summary of the Invention:

The principal object of the present invention therefore is to provide a thermally-responsive device which enables rate sensitive thermal control to be effected mechanically.

According to the present invention there is provided a differential expansion device formed at least in part of synthetic resin material which may, under certain conditions, behave in the same manner as a conventional bimetal formed of two or more metals of different thermal expansion coefficients, and, under certain other conditions, may behave in a manner dependent upon the rate of temperature change.

When a conventional bimetal is heated, the higher thermal expansion component of the bimetal becomes longer than the lower thermal expansion component, and the stresses thus set up cause the bimetal to bend.

Except under conditions of extremely fast temperature rise, the relatively high thermal conductivity of the constituent metals ensures that the whole of the bimetal is at the same temperature, and thus the instantaneous deflection of the bimetal is relatively independent of the rate of rise of temperature, and can be calculated by the formula: K = 3 (aι-a_a)fT-T_a)

2 x s where K is the curvature, a (1 & 2) are the coefficients of thermal expansion, T-T₀ is the temperature rise, and s is the total thickness, provided that the thicknesses of the bimetal components are in the inverse ratio of the square root of the ratio of their moduli of elasticity.

If, however, the entire bimetal or at least the high expansion component thereof were to be constructed of one or more materials . of low thermal conductivity, then under conditions where the rate of temperature rise was moderate to high and was applied only to the high expansion component, the low expansion component would remain at its low initial temperature and the expansion difference between the two components would be greater. The deflection would become: K = 3(a_,. x (T-T_^l - fa₃ x fTi-Tol)

2 x s where Ti is the reduced temperature of the low expansion component. Under perfect conditions T_x would be equal to To, and the a₂ term would become zero, increasing the curvature.

By the provision of such a low thermal conductivity differential expansion device it would be possible to achieve a given deflection at a lower temperature rise with a high rate of change, compared with the temperature rise required at a low rate of change. The converse is also true; if the heat were to be applied to the low expansion side of such a low thermal conductivity differential expansion device, then at high rates of temperature rise the deflection of the device, may be much reduced or even reversed.

Consider, for example, an embodiment of the present invention configured of two plastics materials of different thermal expansion coefficients in a two layer structure similar to that of a conventional bimetal. A principal difference between such a "biplastics" differential expansion device and a bimetal device resides in the thermal conductivity of the. overall device. Whereas the metal constituents of a bimetal device are good conductors of heat so that a bimetal will be at substantially the same temperature throughout, plastics materials in comparison are relatively poor thermal conductors so that a biplastics device in accordance with the present invention can exhibit a significant temperature differential (for example 10°C) between the side thereof that is subjected to heating and its other side. This characteristic of a biplastics device in accordance with the present invention thus gives rise to the possibility that in a control application a substantially constant rate of temperature rise, such as might be experienced in a water boiling vessel as the water is heated from cold towards its boiling point, will have little effect upon the physical shape of the biplastics device since the rate of change of the water temperature will not be such as to give rise to a significant change in the temperature differential across the biplastics device. However, when the water reaches boiling point and its rate of temperature rise decreases to zero, the temperature differential across the biplastics device will reduce to zero thereby causing the device to change shape.

The terms "high rate of temperature rise" and "low thermal conductivity" are relative and it should be understood that the effects abovementioned would exist in all differential expansion devices where the coefficient of expansion of the low expansion component is positive, but would become more significant as the rate of temperature rise increases or the thermal conductivity of the components of the device reduces.

The present invention proposes to employ this effect by manufacture of a thermally-responsive differential expansion device from polymers. While the invention will be explained hereinafter by reference to an exemplary embodiment constructed as a biplastics device, that is to say a device similar to a bimetal but utilizing plastics materials rather than metals, the invention is not restricted to such biplasti.cs devices. An alternative embodiment of the present invention might for example comprise a single material of low thermal conductivity such that differential temperatures will be obtained on opposite sides of the device. Furthermore, a plastics device in accordance with the present invention might incorporate a metallic inter-layer such as for example to enable the device to be self heating by passage of through currents; a similar effect might be obtained by use of an electrically conductive plastics material (such as a polymer loaded with graphite) in a differential expansion device according to the present invention. Yet further possibilities arise from the use of fibre- reinforced, plastics materials which have a coefficient of expansion which is dependent upon the direction of fibre orientation, and also from the temperature dependence of the coefficient of expansion of such materials.

The above and further features of the present invention are set forth with particularity in the appended claims and, together with the advantages thereof, will be better appreciated from consideration of the following detailed description given with reference to the accompanying drawings.

Description of the Drawings:

Figure 1 illustrates schematically an exemplary "biplastics" device in accordance with the teachings of the present invention; and

Figure 2 shows graphs illustrating the effects of temperature and fibre orientation direction, upon the coefficient of linear thermal expansion of a fibre reinforced Polyphenylene Sulphide (PPS) plastics materials.

Description of the Embodiments: The particular embodiment hereinafter described with reference to Figure 1 comprises a biplastics device consisting of a layer 1 of Polybutylene Terephthalate (PBT) and a layer 2 of Polyethylene Terephthalate (PET), both of these materials being readily available thermoplastic polyesters having suitable mechanical and thermal properties when reinforced with, for example, glass fibre. In particular: PBT PET

^' Coefficient of thermal expansion: (E-5) Flexural modulus: (GPa) Tensile strength: (MPa)

Heat deflection temperature: °C @1.8MPa 190 224

By analogy with bimetallic materials, the optimum thickness ratio for a PBT/PET bilayer device is defined by:

where E (1&2) are the respective moduli, 11.7 and 8 GPa, so

The specific curvature, k, for such a device is defined by: k = 3(aι - a=)/2

= 3(7xl0-⁵ - 2.5xl0"^s)/2 = 0.75x10-* This compares with a figure of 0.395x10-* for a typical high flexivity bimetal.

As an example of the design of a component, for example a fire detector, the following calculation shows how one might achieve a 2mm deflection from a temperature rise of 55°C, and what working force might be expected. To obtain a free deflection of 2mm from a temperature rise of 55°C, using a strip of 2mm thickness, which is as thin as may be conveniently moulded, the necessary length may be calculated from the equation: k = 2 x A x s

(T-To) (L²+A²) where A = the deflection, 2mm, s is the thickness,

2mm, L is the required length, and T-To is 55°C. This gives a value of L of about 44mm.

It is desirable to restrict the working deflection to half the free deflection, to obtain optimum output performance. Thus the force which would be generated by such a PBT/PET device may be calculated using standard cantilever beam formulae. The effective modulus may be calculated from the moduli of the two components, in this case, 9.59 GPa. Thus a cantilever beam, 44mm long, 2mm thick, and for example, 10mm wide, would require the following force for a-deflection of 1mm:

P = Exbxs³xA' (A' is the deflection, 1mm) 4xL³ = 2.25Newton.

The internal stress of such a bimetal beam due to thermal expansion is given by:

Stress = 2_ x a x (T-To) x E 3 where: a = k/2 = 0.375xl0-⁴ E = 9.59 GPa T-To = 55°C so that the internal stress = 13.18 MPa, an acceptable figure.

Normal beam theory gives a maximum stress at the outer surface of 10.05MPa, due to bending, and this stress tends to counteract the thermal stresses at the surface, whilst not affecting the thermal stresses at the interface.

Consider now the case for such a PBT/PET bilayer device where the rate of temperature- rise is sufficient to heat the high expansion component, while leaving the temperature of the low expansion component substantially constant; for this case the low expansion coefficient term may be considered to be zero.

In this case the effective specific deflection would be: k' = 3(7xl0-^s)/2 = 1.05xl0-⁴

The temperature rise under these conditions to give the same 2mm deflection would be about 40°C, an idealised gain of about 15°C, or 27%. In practical conditions one would expect the deflection to be achieved with a temperature rise of between 40 and

55°C, depending on the rate of rise.

Possible applications of a device according to the present invention are, for example, as follows:

1) In a fire detection system, where the deflection would be used to operate a fire alarm and extinguishing system. In such a case a rapid rise caused by a local fire would cause a large temperature differential to be quickly established across the device thereby causing earlier functioning of the alarm than would be -the case with a conventional bimetal, whilst a slow rise caused by a more remote or smaller fire would still cause the system to function in similar manner to a conventional bimetal operated system.

2) As a kettle element protector, where the rapid rate of rise of temperature caused by energising the kettle element without sufficient water in the appliance would cause earlier cut¬ off than is possible with conventional bimetal designs, since these have to be set to operate at a temperature high enough to avoid nuisance operation during normal boiling. It would also be possible to detect the reduction in rate of rise when the kettle boils, by making use of the reverse effect, hence giving automatic kettle operation without the need to allow steam to reach the actuator. A thermally sensitive device according to the present invention could be manufactured from any two suitable materials with appropriate mechanical and thermal properties. Polyphenylene Sulphide (PPS, low) and Nylon 4-6 (PA 4-6, high) would provide enhanced performance at high temperatures. The polymer could be used with a metal to provide, for example electrical conductivity. A metal inter-layer could be provided, and this may be of various values of electrical resistance to allow direct heating of the component by varying amounts by the passage of electric current. By using a material of somewhat higher coefficient of expansion for the low expansion component than that in the example, the difference between the effects at low and high rates of rise may be increased, at the expense of the effect at low rates of rise.

In order to function, it is only necessary that the constituent parts of a device in accordance with the invention are securely joined together at their extremities. However, for most purposes it is proposed that the parts be securely joined along the whole of their adjacent faces. This joint could be adhesive, either by the use of a suitable adhesive layer or by a moulding process which moulds one part to a second in such a way that welding of the two takes place. The joint may also be mechanical, by the use of integrally moulded, or separate, lugs or pegs. The joint may be a combination of any or all of these, to limit or eliminate any relative movement of the adjacent faces of the constituents.

^• It is envisaged that the device may be moulded in a cavity with a removable portion, the first constituent part being moulded with the removable portion in place, and the second constituent part being moulded into that part of the cavity which is exposed by the removal of the removable portion. The removable portion may be shaped to allow the formation of mechanical interlocking features between the two constituent parts. It is further envisaged that it will be possible to form at least one constituent part integrally with other moulded parts of a product making use of this invention, by use of a suitably shaped moulding cavity. In the case of the variant where one constituent is metallic, this part may be formed, for example, by a press tool, and the polymeric component subsequently attached by any suitable assembly means, for example by insert moulding or rivetting. Thinner devices, which will be weaker but more thermally active, can be formed by, for example, extrusion and subsequent joining together, if the sections are unsuitable for injection moulding.

Figure 2 shows how the coefficient of linear expansion of a number of fibre reinforced Polyphenylene Sulphide (PPS) polymeric materials marketed under the trademark RYTON PPS by Phillips ^' Petroleum Chemicals vary as a function of temperature and as a function of fibre orientation. The key in Figure 2 schematically illustrates the moulding of a product with injection of material in the direction of the arrow and serves to define transverse (X) and longitudinal (Y) fibre orientation directions relative to the direction of injection. As can be seen from the graphs the coefficient of linear thermal expansion of the different materials is more or less constant in the transverse (X) direction, and exhibits a pronounced non-linearity in the longitudinal (Y) direction with a significant gradient at or around the 100°C temperature of boiling water. The present invention can take advantage of these characteristics in a biplastics device comprising first and second layers moulded with different directions of reinforcing fibre orientation, for example one layer having an X direction fibre orientation and the other layer having a Y direction fibre orientation. Such a device would give rise to no problems as regards the adherence of the two layers to each other, since the basic polymeric material is the same for each layer, and could be readily moulded in one operation by known injection moulding techniques or if formed of separately moulded components could readily be welded together. Such device would perform similarly to the device described' hereinbefore with reference to Figure 1 with the added feature that the more or less abrupt change in the coefficient of expansion of the material in the Y direction of fibre orientation at the temperature of boiling water could be utilized to effect a switching operation.

The invention having been described in the foregoing with reference to specific embodiments, it is to be appreciated that the invention is not limited to the embodiments described and that modifications and variations can be made without departure from the scope of the invention as defined by the appended claims. For example, as previously mentioned herein a device in accordance with the present invention, or a control incorporating the same, could comprise but a single plastics material of low thermal conductivity, the differential linear expansion of relatively hot and cold portions of a member formed of such material being arranged to effect a control function.

Claims

CLAIMS :

1. A thermally-responsive differential expansion device comprising first and second layers, having different coefficients of linear thermal expansion interconnected with each other so as to exhibit a change in physical shape in response to a temperature change, characterized in that at least one of said layers is formed of a material having a relatively low thermal conductivity whereby the device can sustain a temperature differential across the thickness of said layers.

2. A device as claimed in claim 1 wherein both of said layers are formed of low thermal conductivity materials.

3. A device as claimed in claims 1 or 2 wherein either or both of said layers are formed of non- metallic material.

4. A device as claimed in claim 3 wherein said non- metallic material comprises a synthetic plastics material.

5. A device as claimed in claim 4 wherein the synthetic plastics material of which either or both of said layers is formed comprises an electrically conductive component whereby the respective layer is electrically conductive.

6. A device as claimed in claim 4 or 5 wherein both of said layers are formed of synthetic plastics material incorporating reinforcing fibres and the orientations of the reinforcing fibres in the two layers are in different directions.

7. A device as claimed in claim 6 wherein both of said layers are formed of the same synthetic plastics material and the orientations of the reinforcing fibres in the two layers are in directions generally at 90° to each other.

8. A device as claimed in claim 6 or 7 wherein the coefficient of linear thermal expansion of one of said layers is temperature dependent.

9. A device as claimed in claim 4 or 5 wherein the synthetic, plastics material of which one of said layers is formed incorporates reinforcing fibres orientated in a direction such as to provide the respective layer with a temperature dependent coefficient of linear thermal expansion.

10. A device as claimed in any of claims 3 to 9 incorporating a metallic inter-layer.

11. A thermally-sensitive control incorporating a device as claimed in any of the preceding claims.

12. A control as claimed in claim 11 adapted for use as a boiling control in a liquid heating appliance, the arrangement being such that in use of the control the device can sustain a temperature differential across its thickness during heating of the liquid from cold and will not actuate the control, but upon the liquid obtaining its boiling temperature such temperature differential disappears and causes the device to actuate the control.

13. A control as claimed in claim 11 adapted for use as an element protector in an electrically heated water boiling appliance, the arrangement being such that in use of the control the exposure of the device to the rapid rate of temperature rise occasioned by switching on of the appliance without sufficient water therein will cause such a larσe temperature differential to be established across the thickness of the device as to cause operation of the control.

14. A control . as claimed in claim 11 adapted for use as a fire detector or the like, the arrangement^' being such that in use of the control the exposure of the device to the substantial heat of a fire or the like will establish such a large temperature differential across the thickness of the device as to cause substantially instantaneous operation of the control.

15. A control as claimed in claim II or 12 or 13 or 14 wherein a constituent part of the control is formed integrally with a component part of the thermally- responsive differential expansion device.

16. A thermally-sensitive control comprising a thermally-responsive element having a temperature dependent coefficient of linear thermal expansion exhibiting a greater temperature dependency within a predetermined temperature range, and means responsive to the expansion or contraction of said element for initiating a control function when the temperature enters or leaves said temperature range.

17. A thermally sensitive control comprising a thermally-responsive element exhibiting relatively low thermal conductivity whereby upon non-uniform exposure of said element to a temperature source differential thermal expansion, of said element occurs for actuating the control.

18. The use, in a control situation, of a material having a relatively low thermal conductivity, the differential linear expansion of relatively hot and cold portions of a member formed of such material being arranged to effect a control function.

19. The use, in a control situation, of a material exhibiting a significant temperature dependence in its coefficient of . thermal expansion within a predetermined temperature range.

20. An electrically heated water boiling vessel incorporating a thermally sensitive control as claimed in any of claims 11 to 17.

21. A thermally-responsive differential expansion device, a control incorporating the same or an apparatus incorporating such control, substantially as herein described with reference to the accompanying drawings.